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Fungi in the genus <i>Metarhizium</i> (Hypocreales: Clavicipitaceae) are insect pathogens that can establish as endophytes and can benefit their host plant. In field experiments, we observed a positive correlation between the prevalence of <i>M. robertsii</i> and legume cover crops, and a negative relationship with brassicaceous cover crops and with increasing proportion of cereal rye in mixtures. Here, we report the effects of endophytic <i>M. robertsii</i> on three cover crop species under greenhouse conditions. We inoculated seeds of Austrian winter pea (<i>Pisum sativum</i> L., AWP), cereal rye (<i>Secale cereale</i> L.), and winter canola (<i>Brassica napus</i> L.) with conidia of <i>M. robertsii</i> to assess the effects of endophytic colonization on cover crop growth. We recovered <i>M. robertsii</i> from 59%, 46%, and 39% of seed-inoculated AWP, cereal rye, and canola plants, respectively. Endophytic <i>M. robertsii</i> significantly increased height and above-ground biomass of AWP and cereal rye but did not affect chlorophyll content of any of the cover crop species. Among inoculated plants from which we recovered <i>M. robertsii</i>, above-ground biomass of AWP was positively correlated with the proportion of colonized root but not leaf tissue sections. Our results suggest that winter cover crops may help to conserve <i>Metarhizium</i> spp. in annual cropping systems.